The invention relates generally to the control of AC line voltage, and more specifically to AC voltage regulation apparatus of the switching type.
The use of AC power to operate sensitive electrical and electronic equipment has given rise to a concomitant need to regulate the power in order to protect the equipment against sags or surges in the level of the line voltage provided by an AC power source. Over the years, equipment which uses AC power has become more sophisticated and now includes many computers and processor-based equipments having circuits which are extremely sensitive to variations in power supply level. Such circuits include, for example, volatile memories and high speed logic devices which require well-regulated voltages for reliable operation.
There is a plethora of apparatus available for regulation of AC power, but each apparatus has one or more characteristics limiting its use in modern power regulation applications. Such apparatus includes, for example, ferro-resonant or constant voltage transformers, line regulators, and line conditioners.
Ferro-resonant transformers utilize the magnetic saturation of a transformer core in a tank circuit which is intended to resonate at the power line frequency. Since a given core size of a transformer can be driven only to a certain maximum (saturation) flux density, it will limit, or regulate the output voltage. However, as is known, a ferro-resonant transformer utilizes a resonating tank which is sensitive to frequency transients and power factor loads. Furthermore, the output of such a transformer is current limiting in that the output voltage collapses when turn-on surges of equipment demand high charging currents. If a momentary drop-out occurs in the power input to a ferro-resonant regulator, the regulated output will collapse, and then recover over a number of input voltage cycles with unpredictable overshoots and undershoots of the resonating voltage level. The overshoots can exceed the nominal input line voltage and threaten critical equipment which depends upon the regulated voltage. Finally, the response time to small sags and surges in the input voltage level can consume many cycles of the line voltage.
Line regulators include phase-controlled regulators which utilize solid state switches to control the phase of the input line voltage during portions of the input sine wave to achieve a regulated output voltage. As is known, the regulated voltage waveforms produced by such devices can be severely distorted. Furthermore, since phase-controlled regulators do not regulate the peak voltage of the AC sine wave one must use caution in combining them with electronic equipments whose power supplies utilize peak rectifiers. A peak-clipping regulator absorbs the excess of the AC line voltage as the line voltage sine wave approaches its peak during each half cycle. However, such a regulator has poor efficiency, and its use is limited to low power applications because suitable high-power semiconductors are in limited supply.
AC line conditioners generally employ electronic circuits to compensate for any abnormality in an input AC line voltage waveform. However, the efficiency of such conditioners is poor, limited in many instances to about 35 percent. Another type of electronic regulator utilizes a tap switching technique wherein solid state switches connect a number of input and output taps on a transformer, and where an electronic control circuit compares either the input AC or the output regulated voltage against a reference to determine which of the solid state switches must be activated to obtain a semi-regulated output voltage. In such devices, there is always one solid state switch which must bear the full load current as well as inrush surge currents and overloads which can excessively stress presently available devices. Often, because of the deviation in phase between the AC input line voltage and the load current, several switching devices can be on simultaneously. Consequently, a short circuit can exist between two transformer taps with resultant currents which, at times, can exceed the current ratings of the switching devices. If the timing of the tap switching is controlled to occur at the time when the load current passes through zero, the simultaneous operation of several switching devices can be avoided, but the regulated waveform becomes severely distorted during switch-over. Further, tap switching devices typically have a limited number of taps which limits their ability to closely regulate an input voltage. For example, when the input voltage happens to be near the extreme end of a given range defined by one tap configuration, and the input voltage deviates from its regulated shape by a minute amount, the regulator output will then make a rather large step to the next range, resulting in a sudden surge or sag in the regulated waveform. Finally, as tap-switching regulators customarily utilize saturable transformers, excessive currents, threatening to the solid state switches, can be generated when such a transformer saturates.
Finally, a type of regulator employing a motor- or servo-variable transformer is known in which a feedback servo amplifier senses the regulated voltage and drives the servo to position the arm of the transformer in order to obtain the desired output voltage. However, the response time of such a servo circuit is extremely slow due to the mechanical inertia of the servo mechanism.
It is therefore desirable to have an AC line voltage regulator which can respond quickly to aberrations in the input line voltage caused by the generation of the voltage or by the provision of load current by the regulator.